Method and apparatus for mixing combustion gases

ABSTRACT

A gas mixing device for use with a combustion burner assembly is disclosed. The gas mixing device includes an elongate gas expansion nozzle adapted to be positioned within a burner housing, the gas expansion nozzle defining a gas expansion chamber therein extending from a first end to a second end thereof. An inlet port and an outlet port are defined at the respective ends of the gas expansion nozzle. At least one oxidizing gas passageway formed separately of the gas expansion chamber extends from the first end of the second end thereof, and has an oxidizing gas outlet at the second end of the gas expansion nozzle. At least one fuel gas passageway is also formed separately of the gas expansion chamber and extends from the first end to the second end of the gas expansion nozzle, and has a fuel gas outlet at the second end thereof. An elongate gas inlet nozzle is also positioned within the burner housing and is spaced from the first end of the gas expansion nozzle. A portion of the oxidizing gas is passed through the gas inlet nozzle and into the gas expansion chamber, with another portion of the oxidizing gas being passed through the at least one oxidizing gas passageway. A portion of the fuel gas is also passed into the gas expansion chamber and mixes with the oxidizing gas to form a premixed combustion gas stream, with a portion of the fuel gas also being passed through the at least one gas passageway. The separate oxidizing gas and fuel gas streams jacket and surround the premixed combustion gas stream passed from the mixing device.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This patent application claims priority to U.S. provisional Patent Application Number 60/216,753 filed on Jul. 7, 2000, in the United States Patent and Trademark Office, and to U.S. provisional Patent Application Number 60/222,450 filed on Aug. 8, 2000, in the United States Patent and Trademark Office.

FIELD OF THE INVENTION

[0002] The invention relates in general to combustion burner assemblies of the type used in the production of molten metals, for example copper and aluminum. More particularly, the invention relates to an improved gas mixing device, and a method of mixing combustion gases practiced thereby, adapted for use with a combustion burner assembly.

BACKGROUND OF THE INVENTION

[0003] The use of shaft furnace burners with the type of shaft furnace disclosed in U.S. Pat. No. 3,199,977 to Phillips et al., for melting metals, to include copper and aluminum, is well known. As known to those of skill in the art, the combustion burner of Phillips et al. is commonly referred to as a shaft furnace burner, and more particularly to a premix burner of the type that burns an oxidizing gas and a fuel gas that have been mixed with one another prior to ignition within a combustion burner. When the furnace burner is operated in a slightly reducing mode, i.e., where less oxygen than required for complete combustion is used in the combustion process, as is typically desirable for the operation of a metal melting shaft furnace, the pre-mixed and substantially homogenous oxidizing gas to fuel gas mixture or ratio utilized produces a flame with a relatively high adiabatic flame temperature.

[0004] Although the flame temperature of this type of burner is desirable from the standpoint of melting a metal or metals, the elevated flame temperature also leads to the production of extensive amounts of nitrogen oxide (NO_(x)) in the off gas, i.e., the exhaust or waste gas emitted from the burner, which is undesirable. Additional examples of premix burners are disclosed in U.S. Pat. No. 3,299,940 to Phillips et al., as well as in U.S. Pat. No. 4,536,152 to Little, Jr. et al., respectively.

[0005] Moreover, In a shaft furnace of the type used to melt or refine a metal, it is oftentimes necessary to control the concentration of certain combustion gases, for example oxygen, within the raw molten metal in order to control the mechanical properties of the finished product. The known types of shaft furnace combustion burners normally operate in a combustion region where between 90 to 99 percent of theoretical combustion oxygen is supplied to the burners. This oxygen ratio is controlled strictly by measuring, i.e., sampling, and controlling the products of combustion sampled from the combustion of the premixed oxidizing and fuel gas streams, also known as the combustion gas stream. Deviations on either side of the optimal oxygen range may cause the absorption of excess amounts of oxygen by the molten metal, either from direct exposure to oxygen in lean combustion, or from exposure to the unburned oxygen of the oxidizing gas/oxidizer due to an excessively long combustion flame. A sampled premix burner control method of the type described above is disclosed in U.S. Pat. No. 5,240,494 to Williams et al.

[0006] Shaft furnace combustion burner designs that do not premix the oxidizing gas and the fuel gas prior to the injection of the gases into the burner block are know as nozzle mix or non-premix burners. These types of burners operate with a lower adiabatic flame temperature than that of a premix burner, and thus do not attain the melting efficiency of the premix style burners, although they do reduce the amount of nitrogen oxide resulting from the melting process.

[0007] What is needed, therefore, but seemingly unavailable in the art is a gas mixing device, as well as a gas mixing method practiced thereby, adapted for use with a combustion burner suitable for use in the melting and production of high-quality metals having an oxygen content in the normal process range, and which also allows for the strict control of the combustion chemistry of the combustion burner for the purpose of reducing the amount of nitrogen oxide resulting from the melting process. Accordingly, what is needed is a combustion gas mixing device and control methodology that permits the combustion burner to act as both a premix burner for attaining the desired oxygen content in the flame and metal molten thereby, as well as a nozzle mix or non-premix burner which allows for the control of the burner's combustion chemistry, when and as desired.

SUMMARY OF THE INVENTION

[0008] The present invention provides an improved gas mixing device, as well as a method of mixing combustion gases, for use with a combustion burner that overcome some of the design deficiencies of the prior art. Accordingly, the combustion burner of the present invention comprises a gas mixing device that works in conjunction with a conventional shaft furnace burner. The gas mixing device, as well as the gas mixing and control methods of the invention are suitable for use in the melting and production of high-quality metals requiring an oxygen content in a normal process range, and which also allow for the control of the burner's combustion chemistry so as to reduce the amount of nitrogen oxide resulting from the metal melting process.

[0009] The gas mixing device of the invention therefore operates substantially as a nozzle mix type of burner, with an associated lower flame temperature which results in a lower level of nitrogen oxide production than that of a premix burner, while attaining the performance characteristics of a premix burner. This is achieved by providing a gas mixing device having an elongate gas expansion nozzle adapted to be positioned within a housing provided as a part of a burner assembly. The burner assembly, as known, will have an oxidizing gas supply line in sealed fluid communication with the housing, as well as a fuel gas supply line also in sealed fluid communication with the housing. The gas expansion nozzle has a first end and a spaced second end with a gas expansion chamber defined therein and extending from the first end of the nozzle to the second end thereof. An inlet port is defined at the first end of the gas expansion nozzle, and an outlet port is defined at the second end of the gas expansion nozzle, each of which is in communication with the gas expansion chamber.

[0010] The gas mixing device also includes at least one oxidizing gas passageway formed separately of the gas expansion chamber and extending along the gas expansion nozzle, which is provided, and communicates with at least one oxidizing gas outlet at the second end of the expansion nozzle. At least one fuel gas passageway formed separately of the gas expansion chamber is also provided, extending from the first end to the second end of the gas expansion nozzle. The at least one fuel gas passageway is also provided with at least one fuel gas outlet at the second end of the gas expansion nozzle.

[0011] The gas mixing device also comprises an elongate gas inlet nozzle spaced from the first end of the gas expansion nozzle. The gas inlet nozzle has a first end, a spaced second end, and defines a gas flow passageway therein extending from the first to the second ends thereof, respectively. A gas inlet port is defined at the first end of the gas inlet nozzle and which is in communication with the gas flow passageway. In like fashion, a gas outlet port is defined at the second end of the gas inlet nozzle which is also in communication with the gas flow passageway.

[0012] The gas flow passageway defined within the gas inlet nozzle is formed as a venturi for increasing the velocity of the gas passed therethrough and directed toward the inlet port of the gas expansion nozzle. It is anticipated that the gas outlet port of the gas inlet nozzle will be sized smaller than the inlet port of the gas expansion nozzle so as to minimize, or eliminate, any venturi effect that may occur where the oxidizing gas and the fuel gas enter the inlet port of the gas expansion chamber, as discussed in greater detail below.

[0013] As the gas inlet nozzle is spaced from the gas expansion nozzle within the housing, a gap is defined therebetween which functions as a fuel opening for allowing the fuel gas to be passed from a gas manifold extending at least partially about the gas expansion nozzle into the nozzle. The gas flows through this gap into the gas expansion chamber, mixes with the oxidizing gas, and is then passed from the gas expansion chamber as a premixed combustion gas stream into a downstream mixing head, or mixing zone, provided as a part of the burner assembly.

[0014] The gas inlet nozzle also includes at least one oxidizing gas passageway formed therein, extending from the first end to the second end thereof, which is positioned with respect to the at least one oxidizing gas passageway of the gas expansion nozzle. An elongate tube or pipe is passed through both of the respective gas inlet nozzle and gas expansion nozzle oxidizing gas passageways, such that the two passageways are sealed to one another for allowing the oxidizing gas to flow therethrough, as well as through the gap between the gas inlet nozzle and the gas expansion nozzle without otherwise being mixed with the fuel gas.

[0015] So constructed, the gas inlet nozzle directs at least a portion of the oxidizing gas into the inlet port of the gas expansion nozzle, and also directs at least a portion of the oxidizing gas into the at least one oxidizing gas passageway fashioned within each of the gas inlet nozzle, and the gas expansion nozzle, respectively. The fuel gas supply line, in cooperation with the gas manifold formed about the gas expansion nozzle, ensures that at least a portion of the fuel gas enters the inlet port of the gas expansion nozzle, and also ensures that at least a portion of the fuel gas passes into the at least one fuel gas passageway that extends in the lengthwise direction of the gas expansion nozzle.

[0016] Accordingly, oxidizing gas and fuel gas enter into the inlet port of the gas expansion chamber, and are mixed therein to form the premixed combustion gas flow which is passed from the gas expansion nozzle. Simultaneously, separate oxidizing gas and fuel gas streams, respectively, pass through their respective oxidizing and fuel gas passageways and exit through their respective oxidizing gas and fuel gas outlet ports such that the premixed combustion gas stream, as well as the second oxidizing and fuel gas streams, mix with one another in the mixing head of the device. A premix inner combustion flame core is therefore formed, with the second oxidizing gas and fuel gas streams forming a jacket or envelope about the combustion gas flow stream so that when these gases are ignited at a downstream igniter assembly, the premixed combustion gas stream/inner combustion flame core is jacketed by a flame core formed by the second oxidizing and fuel gas streams, once ignited.

[0017] The above-described method also includes the steps of sampling the premixed combustion gas stream downstream of the gas expansion chamber, and of selectively adjusting any one, or combination, of the first oxidizing gas or first fuel gas streams passed into the gas expansion chamber, the second oxidizing gas stream, and the second fuel gas stream, respectively, for varying the combustion characteristics and performance of the mixed gases.

[0018] The method may also include the steps of matching the oxidizing gas to fuel gas ratio or proportions of the second oxidizing and fuel gas streams, respectively, to the oxidizing gas to fuel gas ratio/proportions of the first oxidizing and fuel gas streams, respectively, of the premixed combustion gas stream.

[0019] A novel method for controlling the combustion of a combustion burner assembly also results from the unique design of this invention, which method includes the steps of combining the first stream of the oxidizing gas with the first stream of the fuel gas, respectively, into the premixed combustion gas stream. Thereafter, a sample of the premixed combustion gas stream is obtained through a sampling device extended into the gas stream. A composition analysis is performed of the sampled premixed combustion gas stream, and an ideal oxidizing gas to fuel gas ratio is then calculated using the current combustion burner operating conditions. The calculated ideal ratio is compared to an actual oxidizing gas to fuel gas ratio, as determined by the composition analysis of the premixed combustion gas stream. Thereafter, the oxidizing gas to fuel gas ratio is regulated by adjusting the pressure of the oxidizing gas passed into the gas expansion chamber. This method or control process is continually repeated during the operation of the combustion burner in order to maintain the ideal ratio of oxidizing gas to fuel gas.

[0020] A feature of the invention, therefore, is that the combustion of the respective second oxidizing and fuel gas streams can be delayed, having the effect of reducing the temperature of the combustion flame, which in turn reduces the nitrogen oxide content of the combustion burner waste gases passed from the combustion burner assembly, and/or the shaft furnace. The methodology of the invention also includes the steps of checking the overall burner combustion ratio by measuring the composition of the combusted and non-combusted gases at a test burner positioned downstream of the mixing head, and then comparing the results thereof to the composition of the premixed combustion gas stream sampled and measured at the outlet of the gas expansion chamber. Moreover, the oxidizing and fuel gases are introduced into the mixing head at substantially the same pressure for the purpose of minimizing the likelihood of there being a back-pressure condition within the gas expansion nozzle.

[0021] It is, therefore, a object of the present invention to provide an improved gas mixing device for use with a combustion burner assembly, as well as an improved method of mixing gases practiced thereby, and also an improved method for controlling the gas mixing operation and the combustion of the gases. It is to this object, as well as the other objects, features, and advantages of the present invention, which will become apparent upon reading the specification, when taken in conjunction with the accompanying drawings, to which the invention is directed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a side elevational view in partial cross section of a combustion burner within which the present invention can be used.

[0023]FIG. 2 is a side elevational view in cross-section of an enlarged portion of the burner shown in FIG. 1.

[0024]FIG. 3 is a side elevational view in cross-section of a preferred embodiment of the gas mixing device of the present invention.

[0025]FIG. 4 is a cross-sectional view taken along line 4-4 of FIG. 3 illustrating a gas manifold.

[0026]FIG. 5 is a cross-sectional view also taken along line 4-4 of FIG. 3 of an alternate embodiment of the gas manifold.

DETAILED DESCRIPTION

[0027] Referring now in detail to the drawings, in which like reference characters indicate like parts throughout the several views, FIGS. 1 and 2 illustrate a known type of combustion burner assembly adapted for use with a shaft furnace of the type illustrated in U.S. Pat. No. 3,199,977 to Phillips et al., the provisions of which are incorporated herein fully by this reference. In particular, the combustion burner assembly 5 of FIGS. 1 and 2 is a premix type of combustion burner, as disclosed in the aforementioned '977 patent to Phillips et al., and is also disclosed in U.S. Pat. No. 3,299,940 also to Phillips et al., as well as to U.S. Pat. No. 4,536,152, to Little, Jr. et al., the provisions of each of which are incorporated herein fully by this reference.

[0028] Referring, therefore, to FIGS. 1 and 2, the combustion burner assembly 5 is illustrated for use with a shaft furnace provided with a furnace wall 7 having a refractory lining 8 and enclosed or encased by a steel shell 9. The combustion burner assembly includes an igniter assembly 11 comprised of an elongate tubular section 12 extending from and in fluid communication with a burner outlet 13 defined within and extending through the furnace wall. A plurality of sampling ports 15 are defined within the tubular section of the igniter assembly, and are used for withdrawing combustion gas samples for measuring the respective oxidizing and fuel gas components thereof.

[0029] The igniter assembly also includes an igniter 16, which can be any type of desired, and known, igniter. An elongate housing 17 extends upstream of the igniter assembly 11, and is in sealed fluid communication with an oxidizing gas supply line 19, and a fuel gas supply line 23. The oxidizing gas supply line is regulated by a valve 20, as well as by an orifice plate 21, as described in greater detail in the aforementioned patents to Phillips et al., as well as to Little, Jr. et al. Additionally, it is anticipated that the combustion burner assembly 5 of FIGS. 1 and 2, as well as the combustion burner assembly 30 of FIGS. 3 through 5, could utilize the control system as described in U.S. Pat. No. 5,240,494 to Williams et al., the provisions of which are incorporated herein fully by this reference.

[0030] The fuel gas supply line 23 extends into a gas manifold 24 which extends about the periphery of the housing 17, and in particular a removable sleeve 25 placed therein. The sleeve 25 defines a mixing chamber or zone 27 within the housing, and is provided with a radially spaced series of fuel gas supply openings or inlets 28 defined therein, such that the fuel gas is passed from the supply line into the fuel gas manifold, and from there through the gas inlets into the mixing zone. The fuel gas stream, denoted by the arrows with the reference character “F,” mixes with the oxidizing gas stream, denoted by the arrows with the reference character “O,” for forming a premixed combustion gas stream, denoted by the reference character “C,” which is passed from the mixing chamber toward the downstream igniter assembly.

[0031] A feature of the known type of combustion burner assembly 5 illustrated in FIGS. 1 and 2 is that the mixing of the oxidizing gas and fuel gas can be controlled with some degree of precision for obtaining the desired combustion gas ratios. This results in a combustion gas flame having a relatively high adiabatic flame temperature, but can also result, as known, in the production of undesirable amounts of nitrogen oxide in the off gas or waste gases passed from the combustion burner assembly, as well as the shaft furnace, or other device with which the combustion burner assembly is used. This type of burner assembly construction is contrasted to those types of combustion burners known as nozzle mix burners, or non-premix burners, which operate with a much lower adiabatic flame temperature than that of a premix burner, but which also result in a lesser emission of nitrogen oxide.

[0032] The present invention is illustrated in FIGS. 3 through 5. In fashion heretofore unknown in the art, the gas mixing device 30 of FIGS. 3 through 5 provides a combustion burner assembly which can function in many ways similar to a premix type of burner, and yet which is also possessed of the characteristics of a nozzle mix burner for providing a high adiabatic flame temperature, and for also reducing nitrogen oxide emissions when contrasted with the known types of premix burners. This is obtained by providing a elongate gas expansion nozzle 31 which is positioned within, and supported along the longitudinal axis, denoted by the reference character “A,” within the housing 17 of the combustion burner assembly. The gas expansion nozzle has a first end 32 and a spaced second end 34, and defines a gas expansion chamber therebetween having an inlet port 36 at the first end of the expansion nozzle, and an outlet port 38 at the second end thereof, each of which is in communication with the gas expansion chamber.

[0033] As illustrated in FIGS. 3 through 5, at least one oxidizing gas passageway 39 is defined within the gas expansion nozzle separately of the gas expansion chamber, and extends from the first end to the second end thereof. As shown in FIGS. 3 through 5, there is at least a pair of radially spaced oxidizing gas passageways defined within the gas expansion nozzle. The number of oxidizing gas passageways defined within or otherwise formed as a part of the gas mixing device 30 may be as desired, and as determined by the combustion gas requirements of the furnace or other device with which the combustion burner assembly will be used.

[0034] Each of the oxidizing gas passageways 39 extends to and is in fluid communication with a respective oxidizing gas outlet 40 formed at the second end of the gas expansion nozzle. These oxidizing gas outlets open into a downstream mixing head or zone 42 which is present externally of the outlet port 38 of the gas expansion nozzle 31. The premixed combustion gas stream formed within the gas expansion chamber, as discussed in greater detail below, is mixed with separate oxidizing gas and fuel gas streams, respectively, as described in greater detail, below.

[0035] Referring to FIG. 3, at least one fuel gas passageway 43 is defined by the gas expansion nozzle in cooperation with the housing 17, which fuel gas passageway extends from the first end to the second end of the gas expansion nozzle. Similar to the oxidizing gas passageways, each of the fuel gas passageways terminates, i.e. is in communication with, a respective fuel gas outlet 44 opening to the mixing head 42.

[0036] Although the oxidizing gas passageway 39 is shown as being defined within the gas expansion nozzle, and the fuel gas passageway 43 is shown as being defined by the gas expansion nozzle in cooperation with the interior sidewall of the housing 17, it is anticipated that these gas passageways can be formed in any fashion as desired, and may be defined entirely within the gas expansion nozzle, may be defined by the gas expansion nozzle in association with the housing in which is it placed, or may be fashioned as separate tubes, pipes, or sealed fluid passageways otherwise extending toward the second end of the gas expansion nozzle, and each of which communicates with an oxidizing gas or fuel gas outlet, respectively.

[0037] Positioned upstream of the gas expansion nozzle, and spaced from the first end thereof, is an elongate gas inlet nozzle 46. The gas inlet nozzle has a first end 47, a spaced second end 48, and defines a continuous gas flow passageway therein extending from the first to the second ends of the nozzle. A gas inlet port 51 is fashioned at the first end of the nozzle, and a gas outlet port 52 is fashioned at the second end of the nozzle, each of which is in communication with the gas flow passageway.

[0038] The gas inlet nozzle also includes at least one oxidizing gas passageway 54 defined therein, and extending from the first end to the second end thereof. As shown in FIG. 3, there are at least a pair of radially spaced oxidizing gas passageways defined within the gas inlet nozzle. As for the gas expansion nozzle 31, there will be a corresponding, as well as any desired number, of oxidizing gas passageways defined within the gas inlet nozzle, each of which extends into sealed fluid communication with a corresponding oxidizing gas passageway provided as a part of the gas expansion nozzle. Accordingly, and as shown in FIGS. 3 through 5, an elongate tube or pipe 55 is passed through each of the respective oxidizing gas passageways 39, 54, such that each corresponding oxidizing gas passageway is in sealed fluid communication with the other.

[0039] As described above, the gas inlet nozzle is spaced from the first end of the gas expansion nozzle by a continuous annular gap 56, which gap defines a fuel gas supply opening leading into, and extending toward the inlet port of the gas expansion chamber defined within the gas expansion nozzle.

[0040] In operation, an oxidizing gas “O” is passed through the oxidizing gas supply line 19 toward, into, and through the gas inlet nozzle 46. This is accomplished by passing at least a portion of the oxidizing gas through the gas flow passageway 50, and then through the inlet port 36 of the gas expansion chamber 35. Another portion of the oxidizing gas, in any desired amount, is passed into the oxidizing gas passageways 54 defined in the gas inlet nozzle, through the elongate tubes 55, and through the oxidizing gas passageways 39 defined within the gas expansion nozzle. The oxidizing gas passed through the oxidizing gas passageways will exit the gas expansion nozzle through the oxidizing gas outlet ports 40, and will be passed toward the mixing zone 42, as described above.

[0041] In similar fashion, the fuel gas “F” is passed through the fuel gas supply line 23 and into the annular fuel gas manifold 24 extending about the first end 32 of the gas expansion nozzle 31, as illustrated in FIGS. 3 through 5. So constructed, at least a portion of the fuel gas is passed through the fuel gas passageway 43 and into the gap 56 for entering the inlet port of the gas expansion chamber, and in so doing, will be mixed with the oxidizing gas being passed from the gas inlet nozzle and into the gas expansion chamber. The oxidizing gas and fuel gas passed into the gas expansion chamber will be thoroughly mixed with one another into a premixed combustion gas stream “C,” as shown in FIG. 3. The premixed combustion gas stream is then passed downstream toward the igniter assembly.

[0042] Simultaneous with the passage of at least some of the fuel gas into the inlet port of the gas expansion chamber, a desired amount of the fuel gas is passed through the respective fuel gas passageways 43, and through the respective fuel gas outlet ports 44 into the gas mixing head or zone 42 formed externally, and downstream, of the outlet port of the gas expansion nozzle. Whereas the oxidizing and fuel gases passed through the gas expansion chamber form the premixed combustion gas stream, the “second” oxidizing gas and fuel gas streams, respectively, passed through their respective oxidizing gas and fuel gas passageways are used in a nozzle-mix fashion for jacketing the premixed combustion gas stream before it is passed into the igniter assembly. The practical effect of this type of combustion gas stream formation is that a two-part flame having an inner flame core comprised of the premixed combustion gas stream is created, with the second oxidizing and fuel gas streams, respectively, combusting and jacketing the inner flame core. This allows for a far greater degree of precision in controlling the gas mixing process, as well as the combustion chemistry, for allowing a relatively high adiabatic flame temperature to be obtained, while also controlling the amount of nitrogen oxide that is emitted from the combustion process.

[0043] A gas sampling device 58, of known construction, is positioned downstream of the outlet port of the gas expansion nozzle such that the gas sampling device measures the respective oxidizing and fuel gases which comprise the premixed combustion gas stream. Positioned further downstream of the gas sampling device, for example approximately 25 inches or so downstream of the mixing head, is a conventional test burner which will sample the combustion gases that have been passed downstream, burn same, and determine the composition of the respective oxidizing and fuel gas ratios therein. Accordingly, using both of the gas sampling and the test burner devices, in association with the described construction of the gas mixing device, a very high degree of control is attainable over the gas mixing process, as well as the combustion process itself, for blending, mixing and maintaining the desired oxidizing gas to fuel gas ratios, all for the purposes of attaining the desired flame temperature and nitrogen oxide measurements.

[0044]FIG. 4 is a cross-sectional view of the gas mixing device taken along line 4-4 of FIG. 3, illustrating the fuel gas manifold 24 which extends about the exterior circumference of the gas expansion nozzle 31. The fuel gas passes through the fuel gas supply line 23 such that it is tangentially directed toward the exterior surface of the gas expansion nozzle, and is passed circumferencially about the exterior of the gas expansion nozzle within the gas manifold. In so doing, the fuel gas is evenly distributed or spread about at least the first end of the gas expansion nozzle for uniformly distributing and supplying fuel to the gas expansion chamber through the gap 56, and from there into the inlet port of the gas expansion chamber.

[0045] In FIG. 5, however, the gas supply line 23 is shown intersecting the longitudinal axis A passed through the housing 17 and the gas expansion nozzle 31, such that a deflector plate 62 is required for directing the fuel gas circumferentially about the exterior surface of the gas expansion nozzle, again for attaining the results of uniformly distributing the fuel gas about the gas expansion nozzle, or at least the first end thereof, and from there passing the fuel gas into the gap 56 and then into the inlet port of the gas expansion chamber.

[0046] As shown in FIGS. 4 and 5, by passing the fuel gas flow through the gas manifold and in a direction which is tangent to the exterior surface or periphery of the gas expansion nozzle, a rotational velocity is imparted to the fuel gas therein. This rotational velocity helps to ensure the desired even pressure distribution of the fuel gas about the circumference of the gas manifold, and results in a more uniform entry of the fuel gas into the gap 56 as well as into the fuel gas passageways 43 of the gas mixing device. Additionally, it is anticipated that the rotational velocity of the fuel gas within the manifold may enhance the capability of the gas mixing apparatus to carry pulverized solids therein, as well as other gases or liquids, all as desired. Also, and as one skilled in the art will appreciate, the fuel gas may be passed through the gas inlet nozzle 46 rather than the oxidizing gas, with the oxidizing gas being passed through the gas manifold 24, and through the gap 56 into the inlet port of the gas expansion chamber, as well as through their separate, and respective, oxidizing and fuel gas passageways, as desired.

[0047] The gas expansion nozzle 31, as well as the gas inlet nozzle 41 may be constructed of any desired material, and preferably of a material which is suitable for use in a high gas flow operation with potentially corrosive gases as well as for use in a heavy duty or severe working environment subject to corrosive forces both internally and externally of the burner assembly. For example, both of the gas expansion nozzle and the gas inlet nozzle may be formed of a ceramic or ceramic coated material, or a metallic material, or of any desired metal or non-metallic material. It is anticipated, however, that the tube 55 passed through the gas inlet nozzle and into the gas expansion nozzle will preferably be formed of a metallic material, and more preferably of a stainless steel for its superior durability and resistance to corrosion. Also, and as shown in FIG. 3, the gas inlet nozzle 41 is sealed to the housing 17, such that all of the oxidizing gas, or other gas(es) must flow therethrough, and then into either the gas expansion chamber or into the gas mixing zone 42 through the oxidizing gas passageways, as desired.

[0048] As described hereinabove, therefore, the gas mixing device of this invention, and primarily the gas expansion nozzle 31 and gas inlet nozzle 46 thereof, replace the interchangeable sleeve 25 of the known combustion assembly 5. The gas flow passageway 50 of the gas inlet nozzle is formed as a venturi for increasing the velocity of the gases passed therethrough. The gas expansion chamber 35, however, is formed to be conical, and may be circular in cross-section, for example as shown in FIGS. 4 and 5, although any desired cross-sectional shape of the gas expansion chamber may be used as designed for attaining the desired gas mixing characteristics therein, i.e. for forming the premixed combustion gas stream. Accordingly, the shape of the gas expansion chamber may be, but is not limited to, a conical shape as shown, or may be parabolic, or hyperparabolic, all as desired.

[0049] The control methodology practiced by the gas mixing device utilizes a sample of the oxidizing gas and the fuel gas, which may include, for example, the premixed combustion gas stream. The control process thus includes the steps of obtaining a sample of the premixed combustion gas stream through the gas sampling device 58. A composition analysis of the sampled premixed combustion gas is then performed by burning the premixed sample and analyzing the products of combustion, or by analyzing the ratio of the oxidizing gas to the fuel gas components of the unburned premixed sample directly, or by performing both steps together. Thereafter, an ideal ratio of the oxidizing gas to fuel gas, under the current burner operating conditions, is calculated by using either predetermined data, or chemical calculations, or both, as desired.

[0050] The ideal ratio so calculated is then compared to the actual oxidizing gas to fuel gas ratio as measured in the composition analysis of the premixed combustion gas stream. The oxidizing gas to fuel gas ratio is then regulated, as required, by adjusting the pressure of the oxidizing gas supply line, or the fuel gas supply line, or both, again as desired. This control process, as described above, is then repeated throughout the operation of the combustion burner to maintain an ideal oxidizing gas to fuel gas ratio.

[0051] Accordingly, during operation of the combustion burner a ceramic water-cooled or heat-resistant metal burner block (not illustrated) provided as a part of the igniter assembly is affixed to the mixing head 42, and combustion of the oxidizing gas and fuel gas occurs in stages therein. As described above, it is anticipated that during combustion first the premixed combustion gas stream passed through the gas expansion chamber will be combusted, and will burn at the center of a two-part flame. Thereafter, the second oxidizing gas and fuel gas streams passed through their separate oxidizing gas and fuel gas passageways, and through their respective outlet ports and into the mixing head 42, will be mixed with one another to form an outer core or jacket about the premixed combustion gas stream. The second gas streams are combusted to form the second or outer jacketing portion of the combustion flame. The result of this process is that the delayed burning of the second oxidizing gas and fuel gas streams reduces the flame temperature, which results in a reduced nitrogen oxide content in the burner exhaust or waste gases. The sampled premixed combustion gas stream, taken at the gas sampling device 58 can also be used to indicate the overall combustion chemistry of the flame produced by the combustion for all of the oxidizing gas and fuel gas supplied to the gas mixing device 30, whether a part of the premixed combustion gas stream or the respective second oxidizing and fuel gas streams passed into the mixing head.

[0052] The ratio or proportions of the unmixed oxidizing gas to fuel gas entering the mixing head 42 through the oxidizing gas outlets 40 and the fuel gas outlets 44, respectively, may be adjusted by selecting an average diameter of the oxidizing gas passageways, and in particular the inner diameter of the tubes 55 passed therethrough, or the fuel gas passageways 43, or by any other form of resistance that may be placed therein to ensure that the ratio/proportions of the unmixed oxidizing gas to fuel gas are equal to the ratio/proportions of the oxidizing gas to fuel gas in the premixed combustion gas stream, as measured by the gas sampling device 58. Moreover, the oxidizing gas pressure relative to the fuel gas pressure, at the oxidizing and fuel gas supply lines, respectively, is regulated, in known fashion. The gap 56 formed between the gas inlet nozzle 46 and the gas expansion nozzle 31 and used to pass the fuel gas into the inlet of the gas expansion chamber may also be spaced as desired for regulating the amount of fuel gas allowed to flow into the inlet of the gas expansion chamber.

[0053] The overall ratio of the premixed combustion gas stream to the non-mixed or second oxidizing gas and fuel gas streams, respectively, is set at the desired proportion for combustion within the igniter assembly. This ratio will be maintained as long as the oxidizing gas and fuel gas pressure differentials at the oxidizing and fuel gas supply lines, respectively, and at the gap 56 between the nozzles, remains unchanged. The overall burner combustion ratio may be checked by using the test burner 59, whereupon the composition of the burned or unburned combustion gases may be undertaken and compared to the results of the same quantities of gases measured in the premixed combustion gas stream at the upstream gas sampling device 58.

[0054] It is anticipated that the introduction of the oxidizing gas and the fuel gases into the gas mixing device 30 will occur at substantially the same pressure for each component, in order to mitigate the effects of any back-pressure encountered at the burner outlet, and extending into the interior of the furnace. For example, in the event of a blockage at the burner outlet due to a solidified process metal, the gas pressures will preclude either component, be it the oxidizing gas or the fuel gas, from flowing into their respective supply lines, or into any portion of the gas mixing device normally occupied by the other one of the combustion gas components, which will thus minimize the creation of a dangerous explosive condition within the gas mixing device 30, and more particularly within the gas expansion nozzle 31, and/or the mixing head 42.

[0055] To allow for the use of relatively identical oxidizing and fuel gas supply pressures, it is desirable that the diameter of the gas outlet port 52 provided as a part of the gas inlet nozzle 46 be sized smaller than the diameter of the inlet port 36 of the gas expansion nozzle 31, or that the center line of the gas flow passageway 56 within the gas inlet nozzle be offset laterally with respect to the center line of the gas expansion chamber 35. By making the diameter of the gas outlet port of the gas inlet nozzle smaller than the inlet port of the gas expansion chamber, or by offsetting the gas flow passageway with respect to the gas expansion chamber, the venturi effect of the gas inlet nozzle in association with the inlet of the gas expansion nozzle will be minimized, if not reduced entirely, while still maintaining the enhanced mixing characteristics created by the device.

[0056] An illustrative application of the present gas mixing device would be in the use of a shaft furnace burner used for melting copper or aluminum. The construction of the gas mixing device 30 as described, allows for taking a sample of the premixed combustion gas stream to be analyzed, and the results used to control the combustion process within the furnace, while the secondary oxidizing and fuel gas flow streams passed through the separate oxidizing and fuel gas passageways, respectively, will burn at a lower flame temperature, resulting in a decrease in the quantity of nitrogen oxide produced by the burner.

[0057] It is also anticipated, for example, that the gas mixing device 30 of the present invention may fit inside existing combustion burner designs, in place of the cylindrical mixing sleeves 25 (FIGS. 1 and 2) presently used therewith. For example, the gas mixing device may be used in a T-thermal type 300 and 400 series burner, and other similar burners used in existing shaft furnace applications. The ability to substitute the present invention in place of the standard mixing sleeves of the known types of burners allows for the retrofitting of the gas mixing device into many existing shaft furnaces without substantial modification of the furnace piping.

[0058] Additionally, it is anticipated that the separate oxidizing gas and fuel gas passageways may be formed such that the combination of the gas expansion nozzle 31 and the gas inlet nozzle 46, alone, forms a convergent or divergent nozzle allowing a portion of the oxidizing gas and the fuel gas mixture to bypass the nozzle as desired. In this embodiment, the gas mixing device will premix all of the oxidizing gas and fuel gas used in the combustion burner, prior to the introduction of the premixed combustion gas stream into the igniter assembly.

[0059] Although several embodiments of the invention have been disclosed in the foregoing specification, it is understood by those skilled in the art that many modifications and other embodiments of the invention will come to mind to which the invention pertains, having the benefit of the teaching presented in the foregoing description and associated drawings. It is thus understood that the invention is not limited to the specific embodiments disclosed hereinabove, and that many modifications and other embodiments are intended to be included within the scope of the invention. Moreover, although specific terms are employed herein, they are used only in a generic and descriptive sense, and not for the purposes of limiting the described invention, and the words “a,” “an,” or “the” can mean one or more, depending upon the context in which the terms are employed. 

I claim:
 1. A gas mixing device for use with a burner assembly, the burner assembly including an elongate housing, an oxidizing gas supply line in fluid communication with the housing, and a fuel gas supply line in fluid communication with the housing, the gas mixing device comprising: an elongate gas expansion nozzle adapted to be positioned within the housing, said nozzle having a first end and a spaced second end; a gas expansion chamber defined within the gas expansion nozzle and extending from the first end of the gas expansion nozzle to the second end thereof; an inlet port at the first end of the gas expansion nozzle in communication with the gas expansion chamber, and an outlet port at the second end of the gas expansion nozzle in communication with the gas expansion chamber; at least one oxidizing gas passageway formed separately of the gas expansion chamber and extending from the first end to the second end of the gas expansion nozzle, with at least one oxidizing gas outlet at the second end of the gas expansion nozzle; and at least one fuel gas passageway formed separately of the gas expansion chamber and extending from the first end to the second end of the gas expansion nozzle, with at least one fuel gas outlet at the second end of the gas expansion nozzle.
 2. The gas mixing device of claim 1, further comprising an elongate gas inlet nozzle spaced from the first end of the gas expansion nozzle, the gas inlet nozzle having a first end and a spaced second end.
 3. The gas mixing device of claim 2, said gas inlet nozzle defining a gas flow passageway therein extending from the first end to the second end thereof, a gas inlet port at the first end of the gas inlet nozzle in communication with the gas flow passageway, and a gas outlet port at the second end of the gas inlet nozzle in communication with the gas flow passageway.
 4. The gas mixing device of claim 3, wherein the gas inlet port of the gas inlet nozzle is formed to be larger than the gas outlet port thereof.
 5. The gas mixing device of claim 2, further comprising a corresponding at least one oxidizing gas passageway formed within the gas inlet nozzle and extending from the first end to the second end thereof.
 6. The gas mixing device of claim 5, further comprising an elongate tube positioned within the at least one oxidizing gas passageway of the gas inlet nozzle and extending into the at least one oxidizing gas passageway of the gas expansion nozzle.
 7. The gas mixing device of claim 2, wherein the gas inlet nozzle is adapted to be sealed to the housing upstream of the gas expansion nozzle.
 8. The gas mixing device of claim 2, wherein the gas inlet nozzle directs at least a portion of the oxidizing gas into the inlet port of the gas expansion nozzle.
 9. The gas mixing device of claim 2, wherein the gas inlet nozzle directs at least a portion of the oxidizing gas into the at least one oxidizing gas passageway of the gas expansion nozzle.
 10. The gas mixing device of claim 2, wherein the gas inlet nozzle is constructed and arranged to increase the velocity of the oxidizing gas as it flows therethrough.
 11. The gas mixing device of claim 1, further comprising at least a pair of oxidizing gas passageways spaced radially from one another about the housing axis.
 12. The gas mixing device of claim 1, the gas expansion chamber being formed as a conical expansion chamber opening toward the outlet port of the gas expansion nozzle.
 13. The gas mixing device of claim 1, further comprising an elongate gas inlet nozzle spaced from the first end of the gas expansion nozzle, the gas inlet nozzle having a first end, a spaced second end, a gas flow passageway defined therein and extending from the first end to the second end thereof, a gas inlet port at the first end of the gas inlet nozzle in communication with the gas flow passageway, and a gas outlet port at the second end of the gas inlet nozzle in communication with the gas flow passageway.
 14. The gas mixing device of claim 13, wherein the gas outlet port at the second end of the gas inlet nozzle is sized smaller than the inlet port at the first end of the gas expansion nozzle.
 15. The gas mixing device of claim 1, further comprising a gas manifold extending at least partially about the gas expansion nozzle.
 16. The gas mixing device of claim 15, the gas manifold being constructed and arranged to direct the fuel gas circumferentially about the gas expansion nozzle.
 17. The gas mixing device of claim 1, wherein the gas expansion chamber is circular in cross section about the housing axis.
 18. The gas mixing device of claim 1, further comprising a gas sampling device spaced from the second end of the gas expansion nozzle and positioned with respect to the outlet port of the gas expansion chamber.
 19. The gas mixing device of claim 1, further comprising a test burner spaced from the second end of the gas expansion nozzle, said test burner being constructed and arranged to measure the composition of the mixed gases passed from the gas expansion nozzle, the at least one oxidizing gas passageway, and the at least one fuel gas passageway, respectively.
 20. The gas mixing device of claim 1, wherein a first oxidizing gas stream and a first fuel gas stream, respectively, are passed into the gas expansion chamber and mixed with one another into a premixed combustion gas stream, a second oxidizing gas stream is passed through the at least one oxidizing gas passageway and a second fuel gas stream is passed through the at least one fuel gas passageway, respectively, and the premixed combustion gas stream, the second oxidizing gas stream, and the second fuel gas stream are mixed with one another externally of the gas expansion nozzle.
 21. A gas mixing device for use with a burner assembly, the burner assembly including an elongate housing, a first gas supply line in fluid communication with the housing, and a second gas supply line in fluid communication with the housing, the gas mixing device comprising: an elongate gas expansion nozzle adapted to be positioned within the housing, said nozzle having a first end and a spaced second end; a gas expansion chamber defined within the gas expansion nozzle and extending from the first end to the second end thereof; and an inlet port at the first end of the gas expansion nozzle in communication with the gas expansion chamber, and an outlet port at the second end of the gas expansion nozzle in communication with the gas expansion chamber.
 22. The gas mixing device of claim 21, further comprising at least one first gas passageway formed separately of the gas expansion chamber and extending from the first end of the gas expansion nozzle to at least one first gas outlet port at the second end thereof.
 23. The gas mixing device of claim 22, further comprising an elongate gas inlet nozzle spaced from the first end of the gas expansion nozzle, the gas inlet nozzle having a first end, a spaced second end, a gas flow passageway defined therein and extending from the first end to the second end thereof, a gas inlet port at the first end of the gas inlet nozzle in communication with the gas flow passageway, and a gas outlet port at the second end of the gas inlet nozzle in communication with the gas flow passageway.
 24. The gas mixing device of claim 23, further comprising a corresponding at least one first gas passageway defined within the gas inlet nozzle and extending from the first end to the second end thereof.
 25. The gas mixing device of claim 24, further comprising an elongate tube positioned within the at least one first gas passageway of the gas inlet nozzle and extending into the at least one first gas passageway of the gas expansion nozzle.
 26. The gas mixing device of claim 22, further comprising at least one second gas passageway formed separately of the gas expansion chamber and extending from the first end of the gas expansion nozzle to at least one second gas outlet port at the second end thereof.
 27. The gas mixing device of claim 21, further comprising an elongate gas inlet nozzle spaced from the first end of the gas expansion nozzle, the gas inlet nozzle having a first end, a spaced second end, a gas flow passageway defined therein and extending from the first end to the second end thereof, a gas inlet port at the first end of the gas inlet nozzle in communication with the gas flow passageway, and a gas outlet port at the second end of the gas inlet nozzle in communication with the gas flow passageway.
 28. The gas mixing device of claim 27, wherein the gas outlet port of the gas inlet nozzle is formed to be smaller than the inlet port of the gas expansion nozzle.
 29. The gas mixing device of claim 21, wherein the inlet port of the gas expansion nozzle is formed to be smaller than the outlet port thereof such that the gas expansion chamber opens toward the outlet port of the gas expansion nozzle.
 30. The gas mixing device of claim 21, further comprising a fuel gas manifold extending at least partially about the gas expansion nozzle.
 31. The gas mixing device of claim 30, the fuel gas manifold being constructed and arranged to direct the fuel gas about the gas expansion nozzle.
 32. A gas mixing device for use with a burner assembly, the burner assembly including an elongate housing, a first gas supply line in fluid communication with the housing, and a second gas supply line in fluid communication with the housing, the gas mixing device comprising: an elongate gas expansion nozzle adapted to be positioned within the housing and having a first end, a spaced second end, a gas expansion chamber defined therein and extending from the first end to the second end thereof, an inlet port at the first end thereof in communication with the gas expansion chamber, and an outlet port at the second end thereof in communication with the gas expansion chamber; and an elongate gas inlet nozzle spaced from first end of the gas expansion nozzle, the gas inlet nozzle having a first end, a spaced second end, a gas flow passageway defined therein and extending from the first end to the second end thereof, a gas inlet port at the first end thereof in communication with the gas flow passageway, and a gas outlet port at the second end thereof in communication with the gas flow passageway.
 33. The gas mixing device of claim 32, the gas inlet port of the gas inlet nozzle being formed larger than the gas outlet port thereof, the inlet port of the gas expansion nozzle being formed smaller than the outlet port thereof, and the gas outlet port of the gas inlet valve being formed smaller than the inlet port of the gas expansion nozzle.
 34. A method of mixing combustion gases within a burner assembly, the burner assembly including an elongate housing, an oxidizing gas supply line in fluid communication with the housing, and a fuel gas supply line in fluid communication with the housing, said method comprising the steps of: passing at least a portion of the fuel gas as a first fuel gas stream into an inlet of a gas expansion chamber positioned within the housing; passing at least a portion of the oxidizing gas as a first oxidizing gas stream into the inlet of the gas expansion chamber; combining the first fuel gas stream and the first oxidizing gas stream, respectively, into a premixed combustion gas stream within the gas expansion chamber; passing the premixed combustion gas stream from the gas expansion chamber; passing at least a portion of the oxidizing gas as a second oxidizing gas stream outside of the gas expansion chamber; passing at least a portion of the fuel gas as a second fuel gas stream outside of the gas expansion chamber; and combining the second fuel gas stream and the second oxidizing gas stream, respectively, with the premixed combustion gas stream externally of the gas expansion chamber.
 35. The method of claim 34, including the step of sampling the premixed combustion gas stream downstream of the gas expansion chamber.
 36. The method of claim 34, including the step of selectively adjusting any one or combination of the first oxidizing gas stream, the first fuel gas stream, the second oxidizing gas stream, and the second fuel gas stream, respectively.
 37. The method of claim 34, including the step of matching the oxidizing gas to fuel gas proportions of the unmixed second oxidizing gas and fuel gas streams, respectively, to the oxidizing gas to fuel gas proportions of the first oxidizing gas and fuel gas streams, respectively, forming the premixed combustion gas stream.
 38. The method of claim 34, including the steps of combusting the premixed combustion gas stream for forming a first flame, and then combusting the respective second oxidizing gas and fuel gas streams for forming a second flame that is combined with and jackets the first flame.
 39. The method of claim 34, including the step of distributing the first fuel gas stream at least partially about an elongate gas expansion nozzle in which the gas expansion chamber is defined.
 40. The method of claim 34, including the step of passing a pulverized solid within a selected one of the respective gas streams.
 41. A method of controlling a combustion burner assembly, the burner assembly including an elongate housing, an oxidizing gas supply line in fluid communication with the housing, and a fuel gas supply line in fluid communication with the housing, said method comprising the steps of: combining a first stream of the oxidizing gas and a first stream of the fuel gas, respectively, into a premixed combustion gas stream; obtaining a sample of the premixed combustion gas stream through a sampling device extended into said gas stream; performing a composition analysis of the sampled premixed gas stream; calculating an ideal oxidizing gas to fuel gas ratio under the then current combustion burner operating conditions; comparing the calculated ideal ratio to an actual oxidizing gas to fuel gas ratio as determined by the composition analysis of the premixed combustion gas stream; and regulating the oxidizing gas to fuel gas ratio by adjusting the pressure of the oxidizing gas passed into the gas expansion chamber.
 42. The method of claim 41, the step of performing a composition analysis of the sampled premixed gas stream comprising the step of burning the premixed gas stream sample and then analyzing the products of combustion.
 43. The method of claim 41, the step of performing a composition analysis of the sampled premixed gas stream comprising the step of directly analyzing the oxidizing gas and the fuel gas proportions in the unburned premixed gas stream sample.
 44. The method of claim 41, the step of performing a composition analysis of the sampled premixed gas stream comprising the steps of burning the premixed gas stream sample and analyzing the products of combustion, and then analyzing the oxidizing gas and the fuel gas proportions in the unburned premixed gas stream sample.
 45. The method of claim 41, the step of calculating an ideal oxidizing gas to fuel gas ratio comprising the step of using predetermined data to calculate the ideal ratio.
 46. The method of claim 41, the step of calculating an ideal oxidizing gas to fuel gas ratio comprising the step of using chemical calculations to calculate the ideal ratio.
 47. The method of claim 41, the step of calculating an ideal oxidizing gas to fuel gas ratio comprising the step of using predetermined data and chemical calculations to calculate the ideal ratio.
 48. The method of claim 41, further comprising the step of repeating the process during the operation of the combustion burner for maintaining the ideal ratio.
 49. The method of claim 41, further comprising the step of combusting the premixed combustion gas stream in stages.
 50. The method of claim 41, further comprising the step of combusting the premixed combustion gas stream at the center of a combustion flame exiting from an expansion chamber formed as a part of the combustion burner.
 51. The method of claim 50, further comprising the step of passing a second oxidizing gas stream and a second fuel gas stream into a mixing head, mixing the separate oxidizing gas stream and the separate fuel gas stream together, combusting the oxidizing gas and the fuel gas of said second gas steams, and forming the combusted second gases as an outer jacket about the combustion flame.
 52. The method of claim 51, comprising the step of delaying the combustion of the respective second oxidizing and fuel gas streams with respect to the combustion of the premixed combustion gas stream.
 53. The method of claim 51, comprising the step of using the sampled premixed combustion gas stream to indicate the overall combustion chemistry of the combustion flame produced by the combustion of all of the oxidizing gases and fuel gases comprising the combustion flame.
 54. The method of claim 51, further comprising the step of adjusting the proportions of the second oxidizing gas stream to the second fuel gas stream entering the mixing head so that the second oxidizing gas stream to the second fuel gas stream proportions are equal to the proportions of the oxidizing gas to the fuel gas within the premixed combustion gas stream.
 55. The method of claim 51, further comprising the step of regulating the pressure of the oxidizing gas relative to the pressure of the fuel gas such that the overall ratios of the oxidizing gas to the fuel gas of the premixed combustion gas stream and of the second gas streams, respectively, are at the desired proportions for combustion.
 56. The method of claim 51, further comprising the steps of checking the overall burner combustion ratio by measuring the composition of the combusted and non-combusted gases at a test burner positioned downstream of the mixing head, and then comparing the results thereof to the composition of the premixed combustion gas stream measured at an outlet of the expansion chamber.
 57. The method of claim 51, further comprising the step of introducing the oxidizing gas and the fuel gas into the mixing head at substantially the same pressure for each such gas. 